When an underground quarry or mine has been worked out, there remain underground voids which, in the absence of suitable support measures being taken, run the risk of collapsing and giving rise to ground subsidence which is damaging to surface infrastructure. Therefore, prior to ceasing to monitor cavities, it is appropriate to implement means suitable for avoiding any subsequent disturbance. The present invention relates to such means which are applicable to particularly thick deposits of rock salt (several hundred meters thick), and more precisely to deposits which have been worked by being washed out.
Conventional methods of packing or caving in are not usable in such deposits, because of excavation conditions, washed-out cavities are generally in communication with the surface via a single borehole and they are therefore inaccessible to personnel or to conventional equipment for packing by means of deads. Although they are usually at depths of more than 500 meters, their size of several hundreds of thousands of cubic meters makes them unsuitable for caving in since that will give rise to unacceptable sinking on the surface. This destructive method will normally also give rise to major changes in the flow patterns of underground water, thereby considerably accelerating underground erosion of the salt formation. It is therefore desirable to set up durable support inside the washed-out cavity. At present, the brine present at the end of washing out is used for this purpose. The liquid exerts forces on the wall of salt which are sufficient for stabilizing changes therein over a human life span.
However, this result does not obtain immediately after salt extraction has stopped. The injected fresh water which saturates with dissolved salts on making contact with the walls of salt is at a lower temperature than the surrounding formation, usually by several tens of degrees centigrade at the depths under consideration. This reduces the temperature of the ground surrounding the cavity. After working has stopped, this ground is reheated by geothermal heat flow. Little by little the brine is heated thereby, and consequently it expands. Its volume continues to increase until the temperature of the brine reaches that of the surrounding rock. During this transient period, it is undesirable to seal the cavity. If the cavity is sealed prematurely, the pressure of the brine can increase sufficiently to break the plug of cement closing the borehole. The plug cracks, thereby leaving a path for the brine to reach higher water levels, thereby polluting them.
A washed-out cavity can therefore be abandoned only after thermal equilbrium has been reached both within the rock and between the rock and the brine. In spite of the convection movements in the brine due to the large temperature difference between the ceiling and the sides of the cavity (typically 30.degree. C.) and the resulting improved heat exchange, because of the considerable volumes involved this equilibrium is reached only after a period of many years. Operating companies generally plan to abandon sites after a period of 30 years. During this period of time, the pressure of the brine at the top of the borehole is monitored and it is regularly lowered to below the lithostatic pressure. It is clear that however well such monitoring may be automated, it continues to represent an unwelcome obligation by virtue of its long duration.
It is therefore the object of the present method of abandoning to reduce to a few years the length of time for which monitoring must be continued after the site is no longer worked and the borehole has been sealed. This is achieved by injecting into the cavity quantities of a mixture which is denser than brine and which is capable of setting, while simultaneously withdrawing equivalent quantities of brine from the cavity, and continuing until the cavity has been completely filled with the mixture. As a result, the cavity remains filled at all moments with liquid or solid which applies a supporting force to the cavity walls. There is therefore no danger of a general collapse.
In itself, this method is not new and has been developed more particularly for disposing of waste which is incorporated in the injected mixture. An example is described in published German patent application DE-A-3 141 884. However, this prior document still proposes a method where the cavity filled with mixture is sealed only after a waiting period which corresponds to thermal equilibrium being reestablished at the site. Thus, just as when the cavity is filled with brine, monitoring must be continued for several tens of years. There can be no question of leaving unmonitored an open borehole which extends down to salt-bearing strata which are naturally extremely soluble and which are therefore at the mercy of the slightest infiltration of fresh water.